1. Field of the Invention
The present invention relates to near-field beam shaping of laser beams, particularly high powered laser beams used for machining, marking, materials processing, and alignment. It also relates to beam integration for the purpose of homogenization of light sources to produce a uniform irradiance on an image plane.
2. Description of Related Art
A common method of shaping laser beams into patterns for product marking and machining involves the use of masks. The laser beam impinges upon a mask, which blocks a portion of the laser letting the unblocked portion through the mask resulting in the desired shape. Blocking a portion of the laser removes a portion of the available laser energy and results in inefficient use of the available laser energy, causing increased fabrication time and cost and heating of the mask.
The use of multiple lenses (multi-aperture) to create various shaped illuminations is referred to as multi-aperture beam integration, and the devices performing beam integration are referred to as multi-aperture beam integrators. Recent interest in multi-aperture beam integration has led to the discussion of on-axis multi-aperture beam integration using refractive elements. FIG. 1 illustrates the basic elements of an on-axis multi-aperture beam integrator. Laser light 110 of diameter “D” is incident upon a segmentation array element 120. The segmentation array element 120 has refractive lens arrays, which divide the laser light into multiple beams 140. The refractive lens arrays have refractive lens elements 130 having diameter “d” and focal length “f” (not shown). The multiple beams 140 are incident on a integrator lens 150, which has a focal length “F”, and integrates the multiple beams 140 so that they overlap in a region whose spot size “S” can be expressed as:                     S        =                  F                      f            /            d                                              (        1        )            The difficulty with on-axis, refractive, segmentation array elements is the ability to form complex patterns. For example, to form a ring pattern a conical refractive lens element 130 is needed. Because of etching errors the sharp conical shapes needed are difficult to fabricate. The rounding of sharp edges results in unrefracted light causing stray light irradiance on the image plane instead of the desired pattern.
There are essentially four devices used to form optical patterns with lasers; masks, refractive lenses, reflective lenses and diffractive lenses. Diffraction occurs when light passes through a periodic structure, whose dimensions are comparable to the wavelength of light. When the structure dimensions are much greater than the wavelength then the diffraction effect is less, but refraction and reflection may still occur. Diffraction can be caused by illumination of a periodic structure, where the periodic structure can be thought of as a plurality of periodic sources. Periodic sources interfere with each other allowing various high and low intensity shapes.
FIG. 2 illustrates a simple periodic diffractive surface, in this case plural phase grating structures 210a-d. The periodic structure segments an incoming wavefront and adds a phase tilt to each segment. When the wavefront segments are stitched back together into a continuous wavefront, the phase and direction of travel of the wavefront has been clearly modified by the periodic structure. For this reason, these periodic structures are often referred to as phase gratings. Each phase grating has an associated phase function describing the phase grating's effect on an illuminating beam's phase upon traversing the grating. The grating shown is FIG. 2 is illuminated by an incident laser beam of wavelength λ upon the planar side. As indicated by the grating equation shown in the figure, the amount of local wavefront tilt θ or phase depends on the structure period w. The 0th and 1st order diffraction waves are shown. Structure depth errors and rounding of the sharp corners, which are unavoidable to varying degrees in every manufacturing process, contribute to the presence of the zero-order light. Diffraction gratings provide the ability to generate complicated wavefront shapes and irradiance patterns.
The primary obstacles to using diffraction elements to form complicated shapes are their wavelength dependence and the existence of the 0th order diffracted light on the image plane or region of pattern formation. The first can be mitigated with lower dispersion refractive or reflective elements. The second can be mitigated with tighter fabrication tolerances or using an off-axis design which allows the zero-order light to become spatially separated from the first-order light at the image plane. An obstacle that is typical of multi-aperture beam integrators is their limitation to fixed intensity shapes at fixed distances, not allowing continuous shaping of the intensity pattern. This obstacle is addressed by this patent.
Typical multi-aperture beam integrators use refractive, reflective, or diffractive lenses. For example, Mori et al. (U.S. Pat. No. 5,594,526) shows a projection exposure apparatus forming a plurality of light source images using multi-aperture beam integrators. However, the lenses and lens arrays of Mori are fixed and the patterns created limited to rectangular or circular fixed patterns.